When people look out over the ocean and see rock formations rising from the water or standing on a shoreline, they are observing dramatic results of coastal geology. These features are not random boulders but are the durable remnants of the interaction between land and sea. The presence of these large, often towering, structures indicates a coastline subjected to wave energy and natural erosion. Their existence tells a story of geological resistance, where the hardest materials have outlasted the surrounding rock.
The Mechanics of Coastal Erosion
The creation of these rock structures is driven by distinct processes of marine erosion that reshape the coastline. One powerful force is hydraulic action, which involves waves crashing against a cliff face. This action compresses air trapped within small cracks and fissures in the rock. As the wave retreats, the sudden release of pressure causes the air to expand explosively, weakening the rock structure and dislodging fragments.
Another significant process is abrasion, often described as the “sandpaper effect,” where loose sediment, pebbles, and rock fragments carried by the waves are hurled against the cliff base. This constant grinding action wears away the rock surface and helps to cut a notch at the base of the cliff between the high and low water marks. Chemical weathering also plays a role, especially on rock types like limestone and chalk, where minerals are slowly dissolved by the slightly acidic seawater. These processes relentlessly attack points of weakness, such as joints and faults in the rock.
Identifying Major Coastal Landforms
The large rocks seen along the coast are specific landforms, each representing a different stage in the long-term process of coastal erosion. The process often begins with a headland, which is a resistant mass of rock jutting out into the sea because the softer rock on either side has been eroded away into bays. Waves refract around this headland, concentrating their energy on its sides and seeking out existing cracks.
These initial cracks are enlarged by hydraulic action and abrasion to form a sea cave at the base of the headland. If two caves form on opposite sides of a headland and erode toward each other, they may eventually meet and break through, creating a sea arch. Continued erosion on the base of the arch, combined with weathering on the arch’s roof, will eventually cause the unsupported top section to collapse.
The isolated, vertical column of rock left standing after the collapse of a sea arch is known as a sea stack. These stacks are the quintessential “big rocks” seen off the coast and are characterized by their steep, often sheer sides. Over a much longer period, the stack itself is subjected to the same erosional forces, eventually being worn down until only a low-lying remnant remains, called a stump.
Finally, as the cliff face continually retreats inland due to undercutting and collapse, a wide, flat area of rock is often left exposed at the base of the cliff at low tide. This feature is known as a wave-cut platform.
Why Rock Composition Matters
The location and persistence of these striking landforms are directly tied to the geological makeup of the coastline. The concept of differential erosion explains why these features exist, as it describes the varied rates at which different rock types wear away. Harder, more resistant rocks, such as granite, basalt, or certain types of well-cemented sedimentary rock like chalk or limestone, erode much slower than softer materials like clay or shale.
It is the resistance of the rock that allows it to remain standing as a headland, cave, arch, or stack while the surrounding, less resistant rock is removed. The structure of the rock is also important; features like joints, which are natural fractures, and bedding planes, which are the layers in which the rock was deposited, create lines of weakness. Waves exploit these weaknesses, but the overall hardness of the material is what dictates the long-term stability and prominence of the resulting rock formation.